1. Field of the Invention
This invention relates to harmonic drive transmissions, and more particularly to the manufacture and use of hermetically sealed harmonic drive transmissions.
2. Prior Art
Harmonic drive devices have been utilized for quite a few years, wherever a lightweight, compact, high ratio transmission of power is needed. All harmonic drive transmissions employ three basic elements. Those elements are a circular spline, a flexspline, and a wave generator, assembled normally with the circular spline fixed in a non-rotating arrangement, with the flexspline rotating as the output element. The wave generator, which typically functions as the input element, would be rotated by a primary power source. Teeth on the flexspline and the circular spline are cut to the same circular pitch. However, the flexspline is somewhat smaller in circumference than the circular spline, and it has fewer teeth. The numerical difference in the number of teeth is always a multiple of the number of lobes on the wave generator.
An early example of a harmonic drive transmission, is shown in U.S. Pat. No. 2,969,266 to C. W. Musser, was issued Mar. 22, 1960, the Patent being entitled "StrainWave Gearing Tubular Shaft". Since that time, harmonic drive transmissions have been used for azimuth drives or antenna systems, or rotary cable drives for welding positioners, for valve actuators, for consumer products such as door openers or actuators or automotive closures, for microscope focusing knobs or for steering mechanisms on rocket booster propulsion systems.
There are certain industries, however, where rotary power is needed, yet contamination from any part of the machinery within the worked field would be critical to the work product. The provision of positive, accurately controlled transmission of motion through an uninterrupted metallic barrier is desired, also in those certain fields. Since no packing, bellows or fallible seal is used, some containment of contamination utilizing a harmonic drive transmission, may be obtained. Such a unit is shown in U.S. Pat. No. 3,196,713 to H. A. Robinson, issued in Jul. 27, 1965, entitled "Hermetically Sealed Transmissions". Such a unit may provide typical rotary-to-rotary power utilizing the same three basic components of harmonic drive transmissions, but their configuration has been altered to meet the specific feed through requirement. The flexspline is a flexible walled tube with one end closed by welding its edge to a disc or diaphragm, and the other end of the tube is bolted or welded to a mounting flange. The wave generator, operating inside the stationary flexspline deflects the walls of the tube and flexspline combination into engagement with the circular spline, which functions as the output member and delivers a rotation in the same direction as the input.
It is an object however of the present invention, to provide an improvement over the hermetically sealed transmissions of the prior art harmonic drive units.
It is yet a further object of the present invention, to provide a hermetically sealed harmonic drive transmission which may be utilized in a field where contamination of any sort would be absolutely critical to the product being produced.